x11-libs/gdk-pixbuf: loaders.cache update fixes (thumbnailing, etc)

x11-libs/gdk-pixbuf: loaders.cache update fixes (thumbnailing, etc)

Fix regressions from meson port for gdk-pixbuf loaders.cache updating.

* Add missing gnome2_gdk_pixbuf_savelist call to pkg_preinst for stable
  version, revbumped straight to stable to fix any outdated caches
  for stable users, as it could be causing lack of thumbnailing
  functionality and other issues.
* Add it to ~arch version too, but also downgrade it back to EAPI-7,
  as gnome2-utils.eclass is broken for EAPI-7 still for gdk-pixbuf
  cache updating (missing eutils inherit from emktemp usage and
  broken $ED and $EROOT lack of ending slash handling. Though instead
  of full revert of the changes, convert the ebuilds own $ED and
  $EROOT handling to be EAPI agnostic for now.
* Drop ancient migration log in the ~arch revbump that seems to have
  been for a time when gdk-pixbuf was newly introduced over a decade
  or more ago (when split out of gtk2 in its early days).

Closes: https://bugs.gentoo.org/697602
Package-Manager: Portage-2.3.69, Repoman-2.3.12
Signed-off-by: Mart Raudsepp <leio@gentoo.org>